Method of producing neutron source tube with coated target

ABSTRACT

A method of making an active metal target for a neutron source tube. Deuterium or tritium gas is sorbed into the active metal. The target is then heated and reacted with a coating gas to form a protective coating over the target which prevents desorbtion of the deuterium or tritium during operation of the tube.

0 United States Patent 1151 3,640,597 Noble Feb. 8, 1972 [54] METHOD OFPRODUCING NEUTRON [56] Reference Cited SOURCE TUBE WITH COATED UNITEDSTATES PATENTS TARGET 1,909,916 5/1933 Dnffenbach et al ..l48/6.3 x [72]Inventor: Lowell A. Noble, Hillsborough, Calif, 218471331 12/19582,865,797 l2/l958 ....14s/20.3 [73] Ass1gnee: Varian Associates2,870,339 H1959 .t..250/84.5 2,899,345 8/1959 ..l48/6.3 [22] Ffled' 19642,917,419 12 1959 Robinson ..l48/6.3 211 Appl. No.: 336,075

Primary ExaminerR0dney D. Bennett, J r. Related U.S. Application DataAssistant Examiner-Daniel C. Kaufman [62] Division of Ser. No. 371,803,June 21, 1960. Ammey Rben 57 ABSTRACT 52 u.s.c1 ..316/10, 29/2517,148/63,

50 5 3 2 A method of making an active metal target for a neutron 51 Int.Cl ..H01j 9/38, G21 g 3 04 some tube Deuterium of tritium gas is somethe active 1581 mold of Search ..250 84.5- 313/615' 316/4 metal- Thetarget is heated and reamed with a math"; gas

to form a protective coating over the target which prevents desorbtionof the deuterium or tritium during operation of the tube.

4 Claims, 2 Drawing Figures I l 9 I 1 x 1 1 I INVENTOR LOWELL A NOBLE Wm191a A 7' TOPNE Y METHOD OF PRODUCING NEUTRON SOURCE TUBE WITH COATEDTARGET This application relates specifically to the method aspects ofthe invention and is a division of my copending application Ser. No.371,803, filed June 21,1960.

This invention relates to the art of making vacuum tubes, particularlytubes which provide a source of neutrons.

In general a neutron source tube comprises a vacuumtight envelope filledwith a hydrogen isotope gas (deuterium or tritium), an active metaltarget in the envelope loaded with a hydrogen isotope, and electrodeswhich can be energized to cause the gas to ionize and the ions to strikethe target. When the ions strike the hydrogen isotope in the target, awellknown nuclear reaction occurs which liberates neutrons. It is alsoknown that the yield of neutrons is substantially more when deuteriumreacts with tritium than when deuterium reacts with deuterium, or whentritium reacts with tritium. Conventionally the target is 'made of anactive metal such as titanium which will sorb large quantities ofhydrogen isotopes. In another form the source tube is evacuated insteadof gas filled and contains a supply of hydrogen isotopes which aresorbed in an active metal and can be desorbed and ionized by a sparkdischarge. The ions are then attracted to the target.

One problem associated with neutron source tubes in the past was thatthe pressure and composition of the gas in the envelope varied duringthe life of the tube, causing it to give erratic results.

According to this invention it was determined that the primary reasonwhy the pressure and composition of the gas varied was that over aperiod of time some of the hydrogen isotope gas would be sorbed by thetarget and some of the hydrogen isotope in the target would be desorbedinto the gas. In the case of the modified tube hydrogen would bedesorbed from both the target and the supply metal. It is true that thisaction does reach equilibrium at a given gas pressure, and giventemperature. However, temperature does not remain constant as apractical matter, and the time required to reach equilibrium isrelatively long, particularly at low temperature. Therefore, equilibriumcannot be maintained and the interchange action does occur.

As previously mentioned the greatest neutron yield occurs when unlikehydrogen isotopes react with each other. In the past it was difficult toobtain this optimum neutron yield because if the tube or the supplymetal were filled with one isotope of hydrogen and the target was filledwith another, the isotopes would move from one place to another so thatthe supply metal, target and tube would each contain a mixture ofisotopes, and some deuterium-deuterium or tritium-tritium reactionswould occur which would lower the neutron yield. Even when a singleisotope was used throughout in an attempt to obtain a uniformpredictable neutron yield, the results were erratic because the activemetals would sorb and desorb the hydrogen to cause a change in the gaspressure or vacuum and therefore a change in the operating conditions ofthe tube.

Another feature of the invention which relates to optimum neutron yieldand constant gas pressure or vacuum has to do with the fact that theneutron yield increases with increase in the ratio of hydrogen to activemetal in the target. Prior to this invention it was not possible toutilize the maximum ratio because the hydrogen isotope in the targetwould be desorbed into the tube, particularly in the case of thevacuum-type tube. This of course reduces the ratio of hydrogen to activemetal; and particularly in the case of the vacuum-type tube, causes suchan increase in the pressure in the tube that the tube becomesinoperable.

Thus, an object of this invention is to provide an improved neutronsource tube capable of predictable high neutron yield.

Another object of this invention is to provide a neutron source tubewhich can be stored for long periods of time without detrimental effect.

A further object of the invention is to inhibit the sorption anddesorption of hydrogen by the active metal in a neutron source tube.

By way of brief description the invention involves placing on the activemetal in the tube a coating which can, in the case of the supply metal,be removed to allow desorption, and which can, in the case of thetarget, be penetrated by hydrogen ions.

The invention contains other objects and features of advantage some ofwhich, with the foregoing, will be set forth in the followingdescription of the invention. The invention is not limited to thedisclosed embodiment, as variant embodiments thereof are contemplatedand may be adopted within the scope of the claims.

Referring to the drawing:

' FIG. 1 is an axial cross section ofa gastype neutron source tube.

FIG. 2 is a cross section of an alternate embodiment of one end of thetube in FIG. 1 whereby the structure becomes a vacuum-type neutronsource tube.

Referring to the drawing in detail FIG. 1 shows a gastight envelopecomprising two cup-shaped electrodes 10 and 11 disposed at oppositeends, with a tubular ceramic insulator 12 between the two. A flange 13on the rim of the cup-shaped electrode 10 is sealed Vacuumtight to oneend of the ceramic 12 with ceramic backing ring 14 sealed to the flange13 opposite ceramic 12 to reinforce the bond between metal flange l3 andceramic cylinder 12.

The other end of cylinder 12 is sealed Vacuumtight to the electrode 11with the aid of a metal sealing ring 16 which is are welded to the rimof the electrode 11. The vacuumtight seal between the ring 16 and theceramic cylinder 12 also employs a ceramic backing ring 17 to strengthenthe seal.

A neutron source tube has a target which in this tube is the electrode11 on which a layer 18 of an active metal is disposed on the innersurface as shown, Titanium is the preferred active metal, but of coursethe layer 18 may be made of other active metals, such as zirconium,hafnium, lithium, lanthanum, yttrium, or thorium. A hydrogen isotope,either deuterium or tritium is sorbed by the active metal in accordancewith a standard process such as first baking-out and outgassing the tubeby heating the tube and forming a vacuum therein by fixing a suitablevacuum pump (not shown) to a tubulation 19 which communicates with theinterior of the tube through the electrode l0, and then supplyinghydrogen isotope gas into the tube through the tubulation as the tube iscooled. There are other processes for sorbing hydrogen within an activemetal and the invention is not limited to the particular processdescribed.

After the tube has been outgassed and the active metal has been loadedwith a hydrogen isotope, the standard practice was then to obtain thedesired pressure of hydrogen isotope gas in the tube and finally sealthe tube by pinching off tubulation 19.

According to this invention, however, after the optimum amount ofhydrogen isotope is sorbed by the titanium 18, the hydrogen is removedthrough the tubulation l9 and oxygen is supplied through the tubulationat a pressure preferably between 200 and 400 millimeters of mercury forabout l hour duration at a temperature range of l50200 C. Theseparameters produce an oxide coating or layer 21 on the titanium which isimpervious to hydrogen isotope molecules because the oxide forms a tightand adherent coating on the titanium. Other gases such as nitrogen,chlorine, carbon vapor, or gases containing carbon can be substitutedfor oxygen. The main requirement is that the gases must react with theactive metal to form the impervious thin coating at a temperature ofabout 200 C. or under so that the hydrogen isotope is not desorbed fromthe active metal during the formation of the impervious coating. Afterthe loaded target has thus been protected by the impervious coating, theoxygen is removed through tubulation 19, the desired hydrogen isotopegas is supplied through the tubulation, and the tube is sealed bypinching-off" the tubulation 19.

In order to operate the tube of FIG. 1, an external power source (notshown) is connected to electrodes 10 and II to form a potential gradientbetween them which will cause the hydrogen isotope gas to ionize. Thetarget electrode I1 is made negative with respect to electrode so thatthe positive ions will strike the target. When the ions strike thetarget they will penetrate the coating 21 and will strike the hydrogenisotope in the active metal 18 to cause the neutron yielding reactionpreviously described. The coating 21 should not be made too thickbecause the energy required to force ions through the coating willincrease as the thickness of the coating increases.

Thus it will be understood that the invention provides a coating on theactive metal which prevents desorption and further sorption of hydrogenisotopes by the active metal while at the same time being penetrable byhydrogen isotope ions. In this way it is possible to load the activemetal target with the optimum amount of one hydrogen isotope, fill thetube with another hydrogen isotope at optimum pressure, and have theseconditions remain static until the tube is operated.

Referring now to the vacuum-type tube disclosed in FIG. 2, it will beunderstood that this tube has a vacuumtight envelope comprising the samelower structure as shown in FIG. 1 including cylindrical sidewall 12,and elements 11, 16, 17, 18 and 21. The two tubes differ only at theirupper ends where in FIG. 2 a cup-shaped metal member 10 replaces theelectrode 10 of FIG. 1. Member 10 is provided with a flange 13 which ismetallically bonded to the upper end of cylinder 12, and a ceramicbacking ring 14' is bonded to the upper surface of flange 13 toreinforce the seal. The end of member 10' is apertured to receive a tubeof insulating material such as ceramic having a rim portion 26. The topand bottom surfaces of rim 26 are provided with annular metalliccoatings 27 and 28, respectively. Coating 28 is brazed to the end ofmember 10, and a ceramic backing ring 30 is brazed to the inside ofmember 10 to reinforce the seal. A metal tubulation 19 is brazed to theupper metallic coating 27 on rim 26, and a ceraMic backing ring 31 isbrazed on the tubulation for reinforcement. Ceramic tube 25 is coatedwith two spaced metal strips 33 and 34 of active metal such as titanium,zirconium or hafnium. The oxide or other protective coating as describedin connection with FIG. I is represented by 35 on active metal layers 33and 34 in FIG. 2. A narrow strip of metallizing 36 connects active metal34 to the metallizing layer 27 and thus to tubulation I9. Similarly, anarrow strip of metallizing 37 connects active metal 33 to themetallizing layer 28 and thus to member 10'. It should be understoodthat active metal layers 33 and 34 are also narrow strips extendingalong the ceramic tube 25 but not around it.

The vacuum-type tube described in connection with FIG. 2 is processed inexactly the same manner described in connection with the tube of FIG. 1except that instead of introducing a hydrogen isotope gas after theactive metal surfaces have been protected by the impervious coatings 21and 35, the tube is evacuated and then sealed by pinching-off tubulation19. It should be understood that metal layers 18, 34, and 35 are theonly active metals in the tubes of FIGS. 1 and 2, all the other metalparts being nonactive metals.

In order to operate the tube of FIG. 2, an external power source (notshown) is connected to member 10'. the portion of 19 remaining afterpinch-off, and electrode II. A potential difference can thus be obtainedacross active metal layers 33 and 34 to cause a spark between them,which spark will destroy the coating 35 causing desorption of thehydrogen isotope in the layers 33 and 34. The spark will also causeionization of the desorbed isotope. As in the case of FIG. 1, the target11 of the vacuum-type tube is negative so that ions will strike it tocause the neutron yielding reaction. Although it is preferred to havethe spark between two loaded metal areas 33 and 34, it will beunderstood that the nonactive metallizing strip 36 or 37 could beextended to replace area 34 or 33 so that only one of the sparkelectrodes would be a loaded active metal.

Having thus described the invention, what is claimed as new and desiredto be securedby Letters Patent is:

l. The method of making a neutron source tube having an envelopecontaining an active metal in which is sorbed a hydrogen isotope, saidmethod comprising the steps ofsorbing the isotope in the active metal byfilling the envelope with the hydrogen isotope in the form of gas,thermally shocking said active metal while said gas is present in theenvelope, then removing the hydrogen isotope gas, introducing into theenvelope a coating gas chosen from the group consisting of oxygen,nitrogen, chlorine, and carbon, heating said active metal and coatinggas to a temperature sufficient to cause said coating gas to react withsaid active metal and form a coating thereon which prevents desorptionof the hydrogen isotope in the active metal, removing said coating gas,obtaining another gas content in the envelope, and sealing saidenvelope.

2. The method of processing an active metal to provide a target for aneutron source tube comprising the steps of exposing the active metal toa gas selected from the group consisting of deuterium and tritium,sorbing said gas in said active metal, and then reacting said activemetal with a gas chosen from the group consisting of oxygen, nitrogen,chlorine, and carbon to form a coating on said active metal to seal inthe sorbed gas, said reacting step being performed under conditionswhich prevent desorption ofsaid sorbed gas.

3. The method of making a target for neutron sources. said methodcomprising the steps of sorbing a gas selected from the group consistingof deuterium and tritium in an active metal and then sealing said activemetal with a coating which prevents desorption of said gas and ispenetrable by ions. said sealing step being performed under conditionswhich prevent desorption of said sorbed gas.

4. The method of claim 3 in which said sealing step coniprises exposingsaid active metal to oxygen gas at a pressure of 200 to 400 millimetersof mercury for about one hour at a temperature of to 200 C.

2. The method of processing an active metal to provide a target for aneutron source tube comprising the steps of exposing the active metal toa gas selected from the group consisting of deuterium and tritium,sorbing said gas in said active metal, and then reacting said activemetal with a gas chosen from the group consisting of oxygen, nitrogen,chlorine, and carbon to form a coating on said active metal to seal inthe sorbed gas, said reacting step being performed under conditionswhich prevent desorption of said sorbed gas.
 3. The method of making atarget for neutron sources, said method comprising the steps of sorbinga gas selected from the group consisting of deuterium and tritium in anactive metal, and then sealing said active metal with a coating whichprevents desorption of said gas and is penetrable by ions, said sealingstep being performed under conditions which prevent desorption of saidsorbed gas.
 4. The method of claim 3 in which said sealing stepcomprises exposing said active metal to oxygen gas at a pressure of 200to 400 millimeters of mercury for about one hour at a temperature of150* to 200* C.